from the Shales of the Northumberland Coal-field. 79 
_ to the anterior extremity. The whole surface is covered with 
a coating of enamel, which considerably obscures the mark- 
ings. On the exposed extremity the enamel is thickened and 
is very finely granulated, but here it is never perfect, the 
greater portion of the ornamentation being always exposed ; or, 
at least, it is so in my specimen. The under surface of the 
scale is also grooved lengthwise, but with great inequality, 
and the grooves are so strongly and irregularly pitted that the 
ridges which bound them are distinct only at the margins. 
the whole, then, it is evident that the scales of our fish 
differ considerably from those of Dipterus, in which they are 
described as nearly circular when detached; the sculpture or 
ornamentation of the surface also seems to vary. ‘There is 
likewise another and perhaps more important difference found 
in the dental plates. In Ctenodus these plates are what the 
name implies—solid expansions of dense matter apparently 
composed of dentine and bone; and the ridges are equally 
solid outgrowths of the surface of the plates, bearing tubercles 
or Tastictes or, in other words, the plates are covered with 
denticulated ridges. In Dzpterus, on the contrary, the dental 
organs are uniformly spoken of by Hugh Miller as “ patches 
of palatal teeth ’’*; and Agassiz describes the ridges or “ ca- 
rine ’’ of specimens obtained in the Old Red Sandstone of 
Russia as ‘‘ composed of series of imbricated and articulated 
_ teeth” T; and the figures representing them entirely confirm 
the description. In the Ctenodi this-arrangement cannot be 
said to exist, though there are in a few of the species slight 
traces of something of the kind, particularly in C. elegans. 
It may, however, be doubted how far such characters should 
be considered of generic importance. They point out, never- 
theless, the propriety of retaining for the present, at least, 
until more is known respecting them, the generic appellation 
of Ctenodus for our coal-measure Ctenododipterini. 
The body of the Newsham specimen .is not well defined ; 
_ but as far as the general characters can be determined, they 
_ agree with those of Dipterus. The head is apparently round 
_ and short, being about one-fourth the length of the entire fish, 
which is 3 inches long; and it exhibits in a distinct manner 
_ the underside of the basisphenoid and the prasphenoid bones 
_ united together. They assume the form of a rhomboidal plate, 
_ with a flattened process extending from the anterior and pos- 
_ terior angles; the bones bearing the dental plates have been 
displaced. The two opercula, which are each formed of a 
* Op. cit. 
___ + Monographie des Poissons Fossiles du Vieux Grés-rouge, troisiéme 
_ livraison, p. 128, tab. 33. 
